As exemplary assemblies composed of a base and a porous film, Japanese Unexamined Patent Application Publication (JP-A) No. 2000-143848 and JP-A No. 2000-158798 disclose ink receiving sheets prepared by forming a coat containing a resin, and good and poor solvents for the resin, subjecting the coat to dry phase inversion to form a porous layer. The dry layer transition technique induces microphase separation by evaporating the solvents from the coat, and the resin (polymeric compound) constituting the porous layer is limited to those soluble in low-boiling-point good solvents. Accordingly, it is difficult to use, in this technique, polymeric compounds that have large molecular weights and are inherently sparingly soluble in such solvents.
In addition, this technique preferably employs a low-viscosity coating composition so as to dissolve the polymeric compound satisfactorily and to evaporate the solvent rapidly after the formation of the coat. This, however, results in disadvantages. Typically, it is difficult to give a coat with a sufficient thickness; of components of the coat, a component that has not been removed upon evaporation of the solvents remains in the resulting porous layer, and this impedes the use of nonvolatile additives; the structure of the porous layer is significantly affected by heating conditions and environmental conditions during production processes, and this impedes stable production and tends to result in variation in properties of the film (film quality), such as pore diameter, rate of open area, porosity, and thickness.
On the other hand, PCT International Publication Number WO 98/25997 discloses an assembly including a base and a porous layer and prepared by another production technique than above. This production technique produces the assembly by forming a coat on a base by casting (flow casting), and drying the coat through wet phase inversion at high humidity in two steps. This technique helps to stabilize the environmental conditions during production, but it fails to solve the above problems related to the dry phase inversion technique, because even the wet phase inversion fundamentally employs the procedure of drying by heating.
Recently, with increasing packaging densities and decreasing sizes of electric/electronic components such as semiconductor devices, there have been made demands to give finer patterns and finer pitches of conductive interconnections so as to provide higher-density and highly functional packaging of printed circuit boards.
According to a subtractive technique, a representative process for producing printed circuit boards, a circuit board is produced generally by depositing copper foil on an insulator, applying a resist to the copper foil to give a resist film, exposing the resist film to light through a mask for depicting, carrying out developing and then etching, and stripping the resist film (see Patent Documents 1 and 2). In this technique, a side of the copper foil to be in contact with the insulator is generally subjected to surface roughening, so as to increase the adhesion between the insulator and the copper foil. However, when pitches between conductive interconnections are made finer, the surface roughening of the copper foil causes cutting or peeling (delamination) of interconnections upon etching, due to roughness of the copper foil. In contrast, when copper foil without surface roughening is used, etching is conducted satisfactorily, but the adhesion between the insulator and the copper foil is insufficient, and thereby the overall interconnections are likely to delaminate.
When the copper foil has a large thickness, a center part of the copper foil in a thickness direction is overetched in a direction perpendicular to the thickness direction, and the formed interconnections fail to have rectangular profiles. This will cause cutting or delamination of the interconnections when the pitches between interconnections are made finer. Accordingly, copper foil should have a smaller thickness so as to make pitches finer, but it is difficult to produce and handle such thin copper foil. It is therefore believed that the current subtractive technique has limits on reduction of pitches.
In common production methods of circuit boards, interconnections have been formed by printing a paste onto a resinous film such as a PET (poly(ethylene terephthalate)) film or PI (polyimide) film. When fine interconnections are formed, however, this technique causes, for example, linkage between interconnections due typically to print feathering.
JP-A No. Hei 05-85815 discloses a method of producing a glass ceramic base by kneading an alumina powder, a quartz glass (silica glass) powder, and a calcium borosilicate glass powder to give a green sheet, printing a conductive paste onto the green sheet, and firing the resulting article. Although the green sheet of the powder mixture excels in printability of fine interconnections, this method requires a firing process at temperatures near to 1000° C., resulting in increased cost. In addition, the product lacks flexibility and is brittle.
JP-A No. 2006-135090 discloses a method of producing fine interconnections by applying a laser beam to a surface of a base to form a trench pattern, and placing an ink containing a conductive material into the trench pattern. This method, however, requires a laser beam irradiation process, and thereby suffer from low productivity and increased cost. In addition, it requires a sophisticated technique for placing the ink into the trench pattern and is not suitable for mass production.
JP-A No. 2001-298253 discloses a technique of forming a primary layer for printing, by forming a film on copper foil, which film is made of a porous material containing an amine compound including a group of particles having specific particle diameters, in order to carry out fine printing. According to this technique, however, the resulting primary layer has low strength and is not usable as an interconnection material, because the layer is formed by the application of particles.    [Patent Document 1] JP-A No. 2000-143848    [Patent Document 2] JP-A No. 2000-158798    [Patent Document 3] PCT International Publication Number WO 98/25997    [Patent Document 4] JP-A No. 2003-243799    [Patent Document 5] JP-A No. 2004-63575    [Patent Document 6] JP-A No. Hei 05-85815    [Patent Document 7] JP-A No. 2006-135090    [Patent Document 8] JP-A No. 2001-298253